Neisseria meningitidis is a Gram-negative bacterium which colonizes the human upper respiratory tract and is responsible for worldwide sporadic and cyclical epidemic outbreaks of, most notably, meningitis and sepsis. The attack and morbidity rates are highest in children under 2 years of age. Like other Gram-negative bacteria, Neisseria meningitidis typically possess a cytoplasmic membrane, a peptidoglycan layer, an outer membrane which together with the capsular polysaccharide constitute the bacterial wall, and pili, which project into the outside environment. Encapsulated strains of Neisseria meningitidis are a major cause of bacterial meningitis and septicemia in children and young adults. The prevalence and economic importance of invasive Neisseria meningitidis infections have driven the search for effective vaccines that can confer immunity across different strains, and particularly across genetically diverse group B strains with different serotypes or serosubtypes.
Factor H Binding Protein (fHBP, also referred to in the art as lipoprotein 2086 (Fletcher et al, Infect Immun 2004; 72:2088-2100), Genome-derived Neisserial antigen (GNA) 1870 (Masignani et al. J Exp Med 2003; 197:789-99) or “741”) is an N. meningitidis protein which is expressed in the bacterium as a surface-exposed lipoprotein. Based on sequence analysis of 71 N. meningitidis strains representative of its genetic and geographic diversity, N. meningitidis strains have been sub-divided into three fHBP variant groups (referred to as variant 1 (v.1), variant 2 (v.2), and variant 3 (v.3)) based on amino acid sequence variability and immunologic cross-reactivity (Masignani et al. J Exp Med 2003; 197:789-99). Other workers (Fletcher et al, 2004) have subdivided the protein into two sub-families designated A (which includes v.2 and v.3 of Masignani) and B (v.1). Variant 1 strains account for about 60% of disease-producing group B isolates (Masignani et al. 2003, supra). Within each variant group, there is on the order of about 92% or greater conservation of amino acid sequence. Specifically, conservation within each variant group ranges between 89 and 100%, while between the variant groups (e.g., between v.1 and v.2) the conservation can be as low as 59%. The protein is expressed by all known strains of N. meningitidis. 
Mice immunized with recombinant fHBP developed high serum bactericidal antibody responses against strains expressing fHBP proteins of the homologous variant group (Masignani et al. 2003, supra; Welsch et al. 2004, J Immunol. 172(9):5606-15.). Thus, antiserum prepared against fHBP v.1 confers protection against N. meningitidis strains expressing fHBP v.1, but not against strains expressing fHBP v.2 or v.3. Similarly, antiserum prepared against fHBP v.2 protects against strains expressing v.2 (or v.3) but not v.1 (Masignani et al. J Exp Med 2003, 197:789-99; Beernink et al. J Infect Dis 2007; 195:1472-9). For vaccine purposes, it would be desirable to have a single protein capable of eliciting cross-protective antibodies against fHBP from different variant groups.
Chimeric proteins have been used for vaccine development in a variety of ways. For example, a first strategy employs a genetic or chemical linkage of an antigen to a known, but unrelated, immunogenic protein, such as the diphtheria, tetanus or pertussis toxoid proteins, or the cholera toxin B (CTB) domain, in order to enhance the magnitude of the antibody responses to the antigen of interest. A second strategy uses a genetic fusion of two antigens from the same organism, to enhance cross-protection against strains with antigenic diversity (Giuliani et al. Infect Immun 2005 73:1151-60). An example is the multivalent group B meningococcal recombinant protein vaccine, which contains a mixture of two fusion proteins: a first fusion protein of a GNA2091 protein and a GNA1870 (or “fHBP”) protein, and a second fusion protein of a GNA2132 protein and a GNA1030 protein (Giuliani et al. Proc Natl Acad Sci USA 2006, 103:10834-9). A third strategy has been to construct a fusion of different serologic variants (“serovars”) of one antigen to induce cross-protection against a strains with antigenic diversity. An example is a tetravalent OspC chimeric Lyme disease vaccine, which induced bactericidal antibody responses against spirochete strains expressing each of the OspC types that were incorporated into the construct (Earnhart et al. Vaccine 2007; 25:466-80).
In the examples of chimeric vaccines described that were designed to broaden protective immune responses, the vaccines were composed of repeats of an individual domain with antigenic variability. The respective variants of the domain were expressed in tandem in one protein (i.e., the same domain from different strains, A1-A2-A3-A4, etc). In some cases, these recombinant tandem proteins can be convenient for manufacturing and quality control. However they also can be very large and subject to improper folding or degradation.
One approach to avoiding the problem of large tandem fusion proteins is to design a single polypeptide that is composed of different domains of two antigenic variants e.g., by “swapping” different individual domains of an antigen, or even smaller regions such as individual epitopes from two different proteins, to form a chimeric protein that expresses antigenically unrelated epitopes specific for more than one strain (i.e., different domains from two different strains, A1-B2 or A2-B1, etc.).
This latter approach was undertaken with fHBP. First, in order to facilitate identification of bactericidal regions of fHBP, the protein was divided into three domains, designated A, B and C (Giuliani (2005) Infect. Immun. 73:1151-1160). The A domain is highly conserved across variant groups, whereas the B and C domains contain sequences that diverge among strains. Giuliani et al. identified an fHBP epitope interacting with a bactericidal mAb located in the C domain at R204 (Giuliani (2005) supra). However, a chimeric protein containing the B domain from a variant 3 strain (B3) fused with the C domain of a variant 1 strain (C1) failed to elicit protective bactericidal responses against strains with either v.1 or v.2 fHBP.
Vaccines that exploit the ability of fHBP to elicit bactericidal antibody responses and that can elicit such antibodies that are effective against strains expressing different fHBP variants remain of interest.